Modern homes are filled with sensors. They are used extensively in many major appliances such as washing machines, cloths dryers, dishwashers, water heaters, refrigerators, freezers, ovens and microwave ovens. Growth in the home appliance sensor market is being fueled by end-user demand for smarter and more compact products. Meeting this need has been possible with the development of more compact, accurate, efficient and reliable photo-sensors for replacing conventional mechanical sensors which are generally bulkier, less accurate, less efficient and less reliable.
Advances in color sensors are also helping to meet this need for sensors in appliances. Here, color sensor is defined as a sensor that can simultaneously detect and identify multiple colors. Color sensors can be inexpensive, compact and allow for convenient interfaces with modern control systems. They provide superior readings, enable faster data acquisition and provide more reliable data on the operating conditions of appliances.
U.S. patent application Publication No. U.S. 2004/0135089 to Manz et al. describes a transmission sensor. The sensor measures the turbidity of a liquid by shining light along two paths. One of the paths is a longer path and is detected by a first receiver. The other path is a shorter path and is detected by a second receiver. The light is directly transmitted from a light transmitter, through the fluid media, to the receivers.
FIG. 1 shows another prior-art transmissive device 101 for measuring the turbidity of a liquid media 103, for example dyed water or cloudy water, whereby light 105 is directly transmitted from a phosphor white LED light source 107, through the liquid 103, to a color sensor 109.
The light source 107 and the color sensor 109 are mounted opposite and perpendicularly to each other with a transparent cavity 111 placed between them. The light source 107 illuminates the cavity 111 and the optical characteristics of the transmitted light 105 are determined by the liquid 103 in the cavity.
The liquid 103 can act as a filter. For example, the transmitted light 105 received by the color sensor 109 will appear bluish if the liquid is bluish. That is because the bluish liquid mainly passes the blue light component and absorbs most of the rest.
In an appliance such as a washing machine, the luminance/intensity and chromaticity/color information of light exiting from the fluid is used to determine if any discoloration or contamination has occurred.
However, the transmission sensor described in FIG. 1 has several disadvantages.
The design is very bulky. As shown in FIG. 1, the light source 107 is mounted opposite to the light sensor 109. Therefore, the setup takes up more space and is not practical for applications where space is critical.
The design is complicated and requires difficult routing. The relative positioning between the light source 107, cavity 111 and light sensor is critical and even a slight displacement will affect the accuracy of the result. If, for example, the light sensor 109 is not aligned with the light source 107, the transmitted light 105 detected will not be able to indicate the level or contamination or discoloration correctly.
The transmission sensor design also does not have very good sensitivity for measuring fluids with low contamination or discoloration levels. For a given apparatus size, the transmitted light beam 105 only passes through the liquid 103 once with the effective beam path equal to the distance “d”. The signal attenuation caused by the low contamination level is not able to give a significant or accurate measurement of how discolored or contaminated the liquid is.
The transmission sensor of Manz et al. described above has these same disadvantages and additionally makes no use of modern color sensors or color data.
It would be desirable to have a meter for measuring the turbidity of fluids that is compact, simple to manufacture and has good sensitivity for measuring fluids having low turbidity.